A flat-type strain wave gearing typically comprises a stationary-side internally toothed gear secured so as to not rotate, a driving-side rotatably drivable internally toothed gear, a cylindrical externally toothed gear capable of flexing in the radial direction, and a wave generator for causing the externally toothed gear to flex ellipsoidally. At the two major-axis-direction end parts of the ellipsoidal shape of the externally toothed gear ellipsoidally flexed by the wave generator, the external tooth portions on one side in the tooth trace direction mesh with the stationary-side internally toothed gear and the external tooth portions on the other side in the tooth trace direction mesh with the driving-side internally toothed gear. The number of teeth on the externally toothed gear is the same as the number of teeth on the driving-side internally toothed gear, but is two less than the number of teeth on the stationary-side internally toothed gear.
When the wave generator, which is a rotation-inputting component, rotates, the position at which the stationary-side internally toothed gear meshes with the externally toothed gear and the position at which the driving-side internally toothed gear meshes with the externally toothed gear move in a circumferential direction. In a single, full rotation made by the wave generator, the externally toothed gear rotates by an amount corresponding to the difference in the number of teeth with respect to the stationary-side internally toothed gear. Because the driving-side internally toothed gear has the same number of teeth as the externally toothed gear, the driving-side internally toothed gear rotates integrally with the externally toothed gear. The rotation of the wave generator is reduced in accordance with the difference in the number of teeth, and the reduced rotation is outputted from the driving-side internally toothed gear, which is a reduced-rotation-outputting component.
The wave generator of a flat-type strain wave gearing comprises a wave generator plug having a rigid body of ellipsoidal contour. A wave generator bearing comprising flexible inner/outer races is mounted to the outer peripheral surface of the wave generator plug. Patent Document 1 discloses a wave generator bearing comprising two rows of balls supporting portions of an externally toothed gear that correspond to a stationary-side internally toothed gear and a driving-side internally toothed gear, respectively.
Patent Document 2 discloses a wave generator formed from a pair of ellipsoidal bodies obtained by dividing a wave generator plug in an axial direction, each of the ellipsoidal bodies causing an externally toothed gear to flex ellipsoidally, a wave generator bearing being interposed therebetween. The two ellipsoidal bodies are identically formed, and are set to rotate reciprocally by a prescribed angle.
Patent Document 3 discloses a wave generator in which a wave generator plug is configured from identically formed first and second cams, the wave generator being assembled in a state in which the first and second cams are rotated reciprocally by 90°.
In Patent Document 4, the inventors proposed a plug-shaped curve for defining the ellipsoidal contour of a wave generator plug.